I've made several packs with varying degrees of success. Perhaps my experience will be helpful. The packs I have made have been in the 2600 to 3400 cubic inch volume range. My weight goals have been 10 - 16 ounces. I have included an aluminum "hoop" stay as part of several packs. This adds both weight and comfort.
1). I generally have about 3-5 inches of pack below the bottom of the hip belt attachment. Much more than this and the pack can ride annoyingly on your upper butt. I also design this part of the pack to curve away from the hip belt.
2). My measured torso length is greater than the distance between the shoulder strap attachment point and the top of the hip belt. For me, the difference is 2-3 inches. The shoulder strap attachment needs to be low enough that the straps can curve up over your shoulder before dropping down your chest to the lower attachment. The exact placement of hip and shoulder strap attachments is best determined experimentally
3). I use a simple "J" shape shoulder strap similar to those used on the G4 pack. I think you can get the shape of the G4 straps online. Several things are important with the shoulder straps. First, the space between the straps where they attach to the pack at the top. If you are thick necked you need more distance. Second, the angle of the straps with the mid line of the pack is important. On my most recent pack the departure from vertical is about 3.5" in 12". The length of padding is important too. If the strap padding gets between your torso and your arm it can be irritating as you swing your arms. And, the width of the padding is a consideration. I prefer a narrower strap over some of the wider versions I have made. A wider strap can chafe against your neck and can rub against your arms when walking.

There are several pack patterns available with the designers ideas built in. I have made each of them and have not found any to my complete liking. You might, though, want to take a look at one or another to get some basic ideas. You can get the G4 pattern from Quest Outfitters, an alpine pack design from Rainshed in Corvallis, Oregon, or a RayWay pack kit from Ray Jardine. All have websites.

The pack I just finished is a 3000 cubic inch, 14 oz. version combining what I liked about each of these packs. I also incorporated an aluminum stay similar to that used in the Six Moons Design Starlite pack. Good luck with your planning.